The current wave of 3-dimensional (3D) films is gaining popularity and made possible by the ease of use of 3D digital cinema projection systems. However, the rate of rollout of digital systems is not adequate to keep up with demand, partly because of the relatively high cost involved. Although earlier 3D film systems suffered from various technical difficulties, including mis-configuration, low brightness, and discoloration of the picture, they were considerably less expensive than the digital cinema approach. In the 1980's, a wave of 3D films were shown in the US and elsewhere, making use of a lens and filter designed and patented by Chris Condon (U.S. Pat. No. 4,464,028). Other improvements to Condon were proposed, such as by Lipton in U.S. Pat. No. 5,481,321. Subject matter in both references are herein incorporated by reference in their entirety.
Prior single-projector 3D film systems use a dual lens to simultaneously project left- and right-eye images laid out above and below each other on the same strip of film. These left- and right-eye images are separately encoded (e.g., by distinct polarization or chromatic filters) and projected together onto a screen and are viewed by an audience wearing filter glasses that act as decoders, such that the audience's left eye sees primarily the projected left-eye images, and the right eye sees primarily the projected right-eye images.
However, due to imperfection in one or more components in the projection and viewing system such as encoding filters, decoding filters, or the projection screen (e.g., a linear polarizer in a vertical orientation may pass a certain amount of horizontally polarized light, or a polarization-preserving screen may depolarize a small fraction of the incident light scattering from it), a certain amount of light for projecting right-eye images can become visible to the audience's left eye, and similarly, a certain amount of light used for projecting left-eye images can become visible to the audience's right eye, resulting in crosstalk.
In general, “crosstalk” refers to the phenomenon or behavior of light leakage in a stereoscopic projection system, resulting in a projected image being visible to the wrong eye. Other terminologies used to describe various crosstalk-related parameters include, for example, “crosstalk percentage”, which denotes a measurable quantity relating to the light leakage, e.g., expressed as a percentage or fraction, from one eye's image to the other eye's image and which is a characteristic of a display or projection system; and “crosstalk value”, which refers to an amount of crosstalk expressed in an appropriate brightness-related unit, which is an instance of crosstalk specific to a pair of images displayed by a system. Any crosstalk-related parameters can generally be considered crosstalk information.
The binocular disparities that are characteristic of stereoscopic imagery put objects to be viewed by the left- and right-eyes at horizontally different locations on the screen (and the degree of horizontal separation determines the perception of distance). The effect of crosstalk, when combined with a binocular disparity, results in each eye seeing a bright image of an object in the correct location on the screen, and a dim image (or dimmer than the other image) of the same object at a slightly offset position, resulting in a visual “echo” or “ghost” of the bright image.
Furthermore, these prior art “over-and-under” 3D projection systems exhibit a differential keystoning distortion between the projected left- and right-eye images, i.e., the projected left- and right-eye images have different keystoning distortions, in which each projected image has a magnification that varies across the image such that a rectangular shape is projected as a keystone shape. Furthermore, the left- and right-eye images have different magnifications at the same region of the screen, which is especially apparent at the top and bottom of the screen. This further modifies the positions of the crosstalking images, beyond merely the binocular disparity.
Differential keystoning arises because the ‘over’ lens (typically used for projecting the right-eye image), is located higher above the bottom of the screen than is the ‘under’ lens (used for projecting the left-eye image) and thus, has a greater throw or distance to the bottom of the screen. This results in the right-eye image having a greater magnification towards the bottom of the screen than the left-eye image. Similarly, the left-eye image (projected through the ‘under’ lens) undergoes greater magnification at the top of the screen than does the right-eye image.
This differential keystoning produces two detrimental effects for 3D projection using the dual-lens configuration. First, in the top-left region of the screen, the greater-magnified left-eye image appears more to the left than the lesser-magnified right-eye image. This corresponds in 3D to objects in the image being farther away. The opposite takes place in the top-right region, where the greater-magnified left-eye image appears more to the right and, since the audience's eyes are more converged as a result, the objects there appear nearer. For similar reasons, the bottom-left region of the screen displays objects closer than desired, and the bottom-right region displays objects farther away than desired. The overall depth distortion is rather potato-chip-like, or saddle shaped, with one pair of opposite corners seeming to be farther away, and the other pair seeming nearer.
Second, differential keystoning causes a vertical misalignment between the left- and right-eye images near the top and bottom of the screen. This misalignment can cause fatigue when viewed for a long time, and detracts from some individuals' ability to comfortably and quickly fuse 3D objects imaged there.
Not only is the combined effect distracting to audiences, but it can also cause eye-strain, and detracts from the 3D presentation.
In digital cinema presentations, Matt Cowan teaches, in US published application 2006/268,104A1, a technique of crosstalk correction that subtracts from the image for one eye a fraction of the image for the other eye, in which the fraction corresponds to the expected crosstalk. This works in digital cinema (and video) or projection systems that do not have differential keystone distortion, e.g., systems that multiplex the left- and right-eye images in the time domain so that the left- and right-eye images are projected from the same physical images along the same optical axis such that the two images overlay each other precisely. However, this approach is inadequate for stereoscopic film projection systems, dual-projector systems or single-projector over-and-under systems that exhibit differential keystone distortion.
Furthermore, application of the Cowan technique to a 3D film can degrade the image, because edges of objects subject to crosstalk compensation are effectively sharpened. This occurs because when a compensation is made for a crosstalk that actually occurs at a different location (e.g., due to uncompensated differential distortion), instead of a decreased brightness at the proper location suffering from the crosstalk, a nearby location or pixel has its brightness decreased while the crosstalk remains unaddressed. Thus, instead of merely suffering from uncompensated crosstalk, the result is an artificially dark line near the uncorrected bright line, to produce a visually intensified edge. Thus, a different crosstalk compensation technique is needed in the presence of differential distortion.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings are not to scale, and one or more features may be expanded or reduced for clarity.